1. Field of the Invention
The present invention relates to a piezoelectric ceramic composition used for a surface acoustic wave device. In particular, the present invention relates to a piezoelectric ceramic composition for a surface acoustic wave device which can improve the impedance ratio and coupling coefficient and such a surface acoustic wave device.
2. Description of the Related Art
In recent years, accompanying the progress of mobile communication equipment using high frequencies, components used therein, for example, resonators and filters, have also been required for use in higher frequencies and miniaturization. As the resonators and the filters, surface acoustic wave devices have been used because of advantages in acceleration of use in higher frequencies and miniaturization.
In a surface acoustic wave device, an interdigital transducer (IDT) composed of at least one pair of interdigital electrodes is configured on a piezoelectric substrate, and excitation and reception of the surface acoustic wave are performed by the IDT. As a piezoelectric substrate material of the surface acoustic wave device, a piezoelectric single crystal of, for example, LiTaO3 and LiNbO3, or a piezoelectric ceramic primarily composed of PbTiO3, Pb(Ti,Zr)O3, etc., are used. A laminate in which piezoelectric thin films, such as ZnO thin films, are laminated on an insulation substrate or a piezoelectric substrate is also used as the piezoelectric substrate of the surface acoustic wave device.
When comparisons are made between the piezoelectric single crystal and the piezoelectric ceramic, the speed of sound is lower in the piezoelectric ceramic. Therefore, a piezoelectric substrate made of piezoelectric ceramic is preferable in order to miniaturize the surface acoustic wave device.
Coupling coefficients required of piezoelectric substrate materials are different depending on the intended purposes, although regarding the piezoelectric single crystal, the coupling coefficient is uniquely defined based on the kind and the cut angle. That is, for a surface acoustic wave device using the piezoelectric single crystal, the piezoelectric characteristics and temperature characteristics are uniquely defined based on the kind of the single crystal and the cut angle, so that flexibility in design of devices is reduced.
On the other hand, piezoelectric ceramics, such as Pb(Ti,Zr)O3-based ceramics, have piezoelectric characteristics which can be varied over a wide range by controlling the composition.
However, when the piezoelectric ceramics were used as the piezoelectric substrates of the surface acoustic wave devices in high frequency regions exceeding 10 MHZ, there was a problem in that impedance ratios, that is, the ratios of the impedance at an anti-resonant frequency of Fa to the impedance at a resonant frequency of Fr, are small.
Accordingly, it is an object of the present invention to provide a piezoelectric ceramic composition for a surface acoustic wave device which can achieve a high impedance ratio even in high frequency regions.
The piezoelectric ceramic composition for a surface acoustic wave device is represented by a formula PbaZrxTiy(NimMnnNb⅔)zO3, where:
x+y+z=1;
0.93xe2x89xa6axe2x89xa61.02;
0.32xe2x89xa6xxe2x89xa60.50;
0.41xe2x89xa6yxe2x89xa60.54;
0.03xe2x89xa6zxe2x89xa60.21; and
0.24xe2x89xa6m+nxe2x89xa60.67.
Preferably, m and n fall within the ranges of:
0.46xe2x89xa6m+nxe2x89xa60.67;
0.01xe2x89xa6mxe2x89xa60.66; and
0.01xe2x89xa6nxe2x89xa60.66.
In the piezoelectric ceramic composition for a surface acoustic wave device, at least one element selected from the group consisting of Ba, Ca, and Sr may be substituted for a part of said Pb.
By sintering the piezoelectric ceramic composition, a piezoelectric ceramic suitable for a piezoelectric substrate of a surface acoustic wave device is obtained. The surface acoustic wave device preferably utilizes a SH type surface acoustic wave. In the case, it is preferable that a grain diameter is about 3 xcexcm or less and the sizes of pores and defects in the piezoelectric ceramic are about 3 xcexcm or less. Further, it is preferable that the absolute value of a change rate of resonant frequency with respect to temperature is about 100 ppm/xc2x0C. or less.
By using the piezoelectric ceramic composition for a surface acoustic wave device according to the present invention, excellent impedance ratios can be achieved, higher frequencies can be used and the piezoelectric characteristics can be controlled over a wide range.
Therefore, a surface acoustic wave device which meets use requirements in higher frequencies and miniaturization can be provided with ease according to the present invention.
When m+n is 0.46 or more, but 0.67 or less, m is 0.01 or more but 0.66 or less, and n is 0.01 or more but 0.66 or less, a large electromechanical coupling coefficient can be achieved.
Furthermore, when the crystalline particle diameter is about 3 xcexcm or less, and when sizes of the pores and defects in the sintered material are about 3 xcexcm or less, the impedance ratio can be further improved.